1. Field of the Invention
The invention relates to electronic communications, and more particularly, to recovering a selected channel from an incoming Advanced Television Systems Committee (ATSC) digital television (DTV) signal.
2. Description of the Prior Art
Electronic communication and, in particular, techniques for broadcasting television video signals continue to be developed. Recently, the Advanced Television Systems Committee (ATSC) has introduced the Digital Television Standard. The resulting Digital Television (DTV) system described in the ATSC Digital Television Standard has ushered in a new era in television broadcasting. The impact of DTV is more significant than simply moving from an analog system to a digital system. Rather, DTV permits a level of flexibility wholly unattainable with analog broadcasting. An important element of this flexibility is the ability to expand system functions by building upon the technical foundations specified in ATSC standards such as the ATSC Digital Television Standard (A/53) and the Digital Audio Compression (AC-3) Standard (A/52).
Using conventional NTSC, and its PAL and SECAM counterparts, the video, audio, and some limited data information are conveyed by modulating an RF carrier in such a way that a receiver of relatively simple design can decode and reassemble the various elements of the signal to produce a program consisting of video and audio, and perhaps related data (e.g., closed captioning). As such, a complete program is transmitted by the broadcaster that is essentially in finished form. In the DTV system, however, additional levels of processing are required after the receiver demodulates the RF signal. The receiver processes the digital bit stream extracted from the received signal to yield a collection of program elements (video, audio, and/or data) that match the service(s) that the consumer selected. This selection is made using system and service information that is also transmitted. Audio and video are delivered in digitally compressed form and must be decoded for presentation.
The RF transmission subsystems used in DTV are designed specifically for terrestrial and cable applications. The structure is such that the video audio, and service multiplex/transport subsystems are useful in other applications. In RF transmission, the channel coder takes the digital bit stream and adds additional information that can be used by the receiver to reconstruct the data from the received signal which, due to transmission impairments, may not accurately represent the transmitted signal. The modulation subsystem offers two modes being based on vestigial sideband (VBS) modulation: an 8-VSB mode for terrestrial broadcast, and a 16-VSB mode for high data rates such as cable applications.
FIG. 1 shows a diagram illustrating the nominal VSB channel occupancy of an ATSC DTV signal. The 8-VSB terrestrial broadcast mode is optimized for maximum service area and provides a data payload of 19.4 Mbps in a 6 MHz channel. The 16-VSB high data rate mode, which provides twice the data rate at the cost of reduced robustness for channel degradations such a noise and multipath, provides a data payload of 38.8 Mbps in the single 6 MHz channel. Both modes provide a nominal DTV pilot tone located 310 kHz above the lower channel edge. For example, on channel 45 (656–662 MHz), the nominal pilot tone frequency is 656.310 Mhz. In a DTV transmitter, a modulation unit (or physical layer) uses digital bit stream information to modulate a carrier for the transmitted signal. The DTV receiver must recover this modulated carrier in order to lock to the corresponding 6 MHz channel.
FIG. 2 shows a block diagram of a tuner, intermediate frequency amplifier, and FPLL in the prototype VSB receiver 200 described in the Guide to Use of the ATSC DTV Standard. The operation of the receiver 200 is described in detail on page 88 of the Guide to Use of the ATSC DTV Standard, 4 Dec. 2003, and more specifically, in U.S. Pat. No. 4,072,909, disclosed by Citta and issued on 7 Feb. 1978, which are included herein by reference. As shown in FIG. 2 and described in the above-mentioned documents, the prototype receiver 200 uses pilot carrier components in both the in-phase I and quadrature-phase Q baseband DTV signals for controlling carrier recovery.
When recovering information from a selected channel, the synthesizer 204 generates a reference signal 206, which is mixed in the tuner 208 with the incoming signal S to produce a down-converted signal 210. Theoretically, the SAW filter 202 has an in-band frequency range which perfectly corresponds with the down-converted channel occupancy of an ATSC DTV signal shown in FIG. 1. However, in actual implementations, there are often differences between reference signal frequencies. Therefore, the down-converted channel may be shifted and partially filtered by the SAW filter 202. If the DTV pilot tone located 310 kHz above the lower channel edge is filtered away by the SAW filter 202, the prototype VSB receiver 200 is unable to lock to the channel and recover information from the selected channel.